高速炉燃料被覆管(316SS)の高温ナトリウム中での腐食挙動(4)ヒートの異る管材の引張性質に及ぼす温度勾配下での5,000時間浸漬の影響

Corrosion behavior of the cladding tubes (316SS) for FBR in high temperature sodium (IV); Effect of sodium exposure for 5,000 hours under a temperature gradient on tensile properties of various heats of fuel cladding tubes

柚原 俊一*; 加納 茂機; 中筋 孝

not registered; Kano, Shigeki; Nakasuji, Takashi

316SS燃料被覆管のプトリウム浸漬後の引張性質データを求めることが,燃料集合体の設計・製作及び健全性評価のための基礎資料として要請されている。本報告では,連続的な温度勾配を有する材料試験ループ1のM―2試験部に外国製316SS被覆管4種を含む13種のヒートの「もんじゅ」及び「常陽」用の316SS試作燃料被覆管を約5000時間浸漬し,これらの被覆管の引張性質に及ぼす浸漬温度の影響を調べに,得られた結果を要約すると以下の通りである。1)未浸漬の受け入れ被覆管とナトリウム浸漬後の被覆管の引張特性を比較すると,400から650での温度範囲でのナトリウム浸漬により,耐力及び引張強さは低下し,伸びは増加した,浸漬前後の引張性質の変化の度合は,浸漬温度により異り,500で最も大きかった。2)結晶粒度及び冷間加工度が引張性質の引張試験温度依存性に及ぼす影響は,受け入れ被覆管の場合と比較して,ナトリウム浸漬によっても傾向的には変らない。また微量ホウ素成分はナトリウム浸漬前後の引張性質の変化量を少なくすること,すなわち安定性に寄与していることが考えられる。3)「もんじゅ」製作準備設計2)で燃料集合体の設計にさいして適用されたSy(耐力),Su(引張強さ)値とナトリウム浸漬後の「もんじゅ」用試作被覆管の引張強度データを比較すると,浸漬後の被覆管の耐力及び引張強さはSUS316焼鈍材のSy,Su値よりは上まわるが,SUS31620%冷間加工材のSy,Su値よりは下まわる。従って時間に依存しない設計応力強さSmについても同様なことが云える。4)500以上の温度でナトリウムに浸漬された被覆管は,浸漬後の引張により浸漬表層部で浅い粒界割れが生ずるが,この表層粒界割れは粒界腐食変質層を越えては進展しないものと考えられる。

As the basis for designing and manufacturing the fuel assembly for MONJU, and for confirming its integrity, it has been required to get the data concerning tensile properties of various heats of fuel cladding tubes exposed to different sodium environments. In this report, thirteen heats of fuel cladding tubes containing four kinds of ones manufactured in a few foreign countries were examined. These tubes were exposed to sodium at the linear temperature gradient test section of the Material Test Sodium Loop 1 for about 5000 hours. After that, tension tests of these tubes were made to clarity effects of sudium exposure on these tensile properties. Furthermore, additional tests related to these tensile properties were done. The results obtained are as follows. (1)From comparison of tensile properties before and after sodium exposure of these tubes, yield strength and tensile strength decreased after exposure, and fracture elongation increased. The variation amount of the values on tensile properties depended on exposure temperatures, and it was largest in the case of 500C. (2)The effects of cold-working and grain size on tensile test temperature dependence of tensile properties after exposure to flowing sodium up to 650C were similar to those before exposure in tendency. In respect to the effect of chemical composition, it is considered that minor boron content contributes to enhance the stability of tensile properties for sodium exposure. (3)From comparison of the tensile strength values (yield strength Sy, tensile strength Su) applied for designing the Monju fuel assembly with the present test results after sodium exposure, the experimental values were higher than those uf Sy and Su of annealed SUS316, but lower than those of Sy and Su of 20% cold-worked SUS316. (4)After tension test of sodium exposed tubes within the temperatures of 500C to 650C, many grain boundary cracks in slight depth were round out in the sodium ...